Weld structure having excellent resistance brittle crack propagation resistance and method of welding the weld structure

ABSTRACT

A welding method for manufacturing welded structures having excellent properties to prevent the propagation of brittle fracture occurring in welded joints, characterized by the step of forming a repair weld having a greater toughness than that of a butt weld and an outer edge whose angle φ with respect to the longitudinal direction of the butt weld is not less than 10 degrees and not more than 60 degrees, by applying repair welding to a region to arrest a brittle crack in a butt-welded joint where a brittle crack is likely to propagate after removing part of the butt-welded joint, in said region, by gouging or machining.

TECHNICAL FIELD

The present invention relates to welded structures, having excellentresistance to brittle crack propagation, that prevent the propagation ofbrittle cracks developed in welded joints and to welding methodsproviding such welded structures.

More concretely, the present invention relates to welding methodsproviding welded structures, having excellent resistance to brittlecrack propagation, that prevent the propagation of brittle cracks, thatmight occur in welded joints of welded structures formed by applyinglarge-heat-input welding to steel plates, and to technologies to enhancethe safety of buildings, civil engineering steel and other structures.

BACKGROUND ART

Welding is indispensable for the construction of steel structures andlarge-heat-input welding is extensively applied to cut down constructioncost and increase construction efficiency. Particularly, as steel platethicknesses increase, the time required for welding increases greatlyand, therefore, the need to perform welding with the maximumlarge-heat-input also increases.

However, application of large-heat-input welding lowers the toughness ofthe heat-affected zone (HAZ), increases the width of the HAZ and, as aresult, lowers the toughness against brittle fracture.

Because of this, steels which are resistant to a drop in the fracturetoughness in the HAZ even when large-heat-input welding is applied wereinvented as disclosed, for example, in Japanese Unexamined PatentPublication (Kokai) Nos. 06-88161 and 60-245768.

Having improved fracture toughness or greater resistance to brittlefracture, the steels according to the above inventions seldom undergobrittle fracture under ordinary service conditions. If, however, abrittle fracture occurs in an earthquakes or other accident anddisasters like a collision between structures, there are risks thatbrittle fractures propagate through the HAZ and cause massive fractures.

It has conventionally been considered that, with TMCP steel plates ofapproximately 25 mm in thickness, brittle fracture diverts from weldedjoints to the base metal because of the residual stress in the weldzone. Therefore, it has been considered that brittle cracks occurring inwelded joints would be stopped in the base metal so long as the basemetal has adequate crack arrestability.

With the upsizing of steel structures, thicker steel plates have come tobe used. Because the use of thicker plates is conducive to structuresimplification too, high tensile strength steel plates offering highdesign stresses are finding increasing use.

The full-size breakdown test conducted by the inventors by using an8000-ton large-sized tester revealed that brittle cracks developed inthicker steel plates of the type described above propagate along theheat-affected zone of welded joints without diverting to the base metal.

In the brittle fracture test conducted by the inventors, a stiffener(reinforcement) 3 fillet-welded to a steel plate 1, not greater than 50mm in thickness, so as to intersect a butt-welded joint thereof as shownin FIG. 1 often arrested the propagation of brittle crack developed inthe steel plate 1 and thereby prevented the fracture thereof.

In thicker plates, however, brittle cracks sometimes propagated alongthe HAZ or weld metal without diverting to the base metal despite theprovision of the stiffener 3.

SUMMARY OF THE INVENTION

The object of the present invention is to provide welded structures thatprevent fatal fracture thereof by preventing the propagation of brittlecracks that occur in welded joints at repair welds and methods formanufacturing such welded structures.

The inventors completed the present invention based on a finding thatapplication of specific welding to welded structures prevents thepropagation of brittle cracks that occur in welded joints and can oftenlead to large-scale fracture.

The gist of the present invention is as described below.

(1) A welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fractureoccurring in welded joints, characterized by the step of forming arepair weld having a greater toughness than that of a butt weld and anouter edge whose angle φ, with respect to the longitudinal direction ofthe butt weld, is not less than 10 degrees and not more than 60 degrees,by applying repair welding to a region to arrest a brittle crack in abutt-welded joint where a brittle crack is likely to propagate.

(2) The welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fracturedescribed in (1) above, in which the toughness in said repair weld is atleast 20° C. lower than the brittle-to-ductile fracture transitiontemperature vTrs of the butt-welded joint.

(3) The welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fracturedescribed in (1) or (2) above, in which said repair weld is provided byapplying repair welding to the pertinent region after removing ½ or moreof plate thickness from either or both sides of the butt-welded joint bygouging or machining.

(4) The welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fracturedescribed in any of (1) to (3) above, in which compressive residualstress not less than ½ of the yield stress of the welded member isdeveloped in a direction perpendicular to the longitudinal direction ofthe butt weld in a region where said repair weld and butt-welded jointare in contact.

(5) The welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fracturedescribed in any of (1) to (4) above, in which repair welding is appliedby controlling so that the angle θ of the longitudinal direction of atleast the last layer of the repair weld bead with respect to thelongitudinal direction of the butt weld is not greater than 80 degrees.

(6) The welding method for manufacturing welded structures havingexcellent properties to prevent the propagation of brittle fracturedescribed in any of (1) to (5) above, in which repair welding is appliedby controlling so that the angle φ of the outer edge of the butt weldwith respect to the longitudinal direction of the butt weld is not lessthan 10 degrees and not more than 45 degrees.

(7) A welded structure having excellent properties to prevent thepropagation of brittle fracture occurring in welded joints,characterized by that the welded structure has a repair weld having agreater toughness than that of a butt weld and an outer edge whose angleφ with respect to the longitudinal direction of the butt weld is notless than 10 degrees and not more than 60 degrees in a region to arresta brittle crack in butt-welded joints where a brittle crack is likely topropagate.

(8) The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in (7) above, in which thetoughness in said repair weld is at least 20° C. lower than thebrittle-to-ductile fracture transition temperature vTrs of thebutt-welded joint.

(9) The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in (7) or (8) above, inwhich said repair weld is provided by applying repair welding to thepertinent region after removing ½ or more of plate thickness from eitheror both sides of the butt-welded joint by gouging or machining.

(10) The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in any of (7) to (9) above,in which compressive residual stress not less than ½ of the yield stressof the welded member is developed in a direction perpendicular to thelongitudinal direction of the butt weld in a region where said repairweld and butt-welded joint are in contact.

(11) The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in any of (7) to (10)above, in which the angle θ of the longitudinal direction of at leastthe last layer of the repair weld bead with respect to the longitudinaldirection of the butt weld is not greater than 80 degrees.

(12) The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in any of (7) to (11)above, in which the angle φ of the outer edge of the butt weld withrespect to the longitudinal direction of the butt weld is not less than10 degrees and not more than 45 degrees.

Thus, the present invention prevents the propagation of brittle cracksthat occur in welded joints in welds by applying specific welding topart of butt-welded joints.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a welded structure with a stiffener.

FIG. 2 shows a welded structure with repair welding.

FIG. 3 shows a welding method for welded structures that prevent thepropagation of brittle fracture.

FIG. 4 shows a test specimen taken from an embodiment of the presentinvention.

THE MOST PREFERRED EMBODIMENT

Details of the best mode for practicing the present invention aredescribed by reference to FIGS. 2 to 6.

FIG. 2 shows a butt-welded joint of steel plate to which the weldingmethod of the present invention is applied. In FIG. 2, referencenumerals 2, 5 and 6 respectively designate a butt-welded joint, a buttweld zone, and a repair weld.

The repair weld according to the present invention is a repair weld 6,as shown in FIG. 2, that has a greater toughness than that of a buttweld 5 and an outer edge whose angle φ with respect to the longitudinaldirection of the butt weld is not less than 10 degrees and not more than60 degrees by applying repair welding with a welding material providingexcellent toughness to a region to arrest brittle crack in a butt-weldedjoint 2 (which consists of the butt weld 5 and the heat-affected zone ofsteel plate 1) where brittle crack is likely to propagate after removingpart of the butt-welded joint 2 (which consists of the butt weld 5 andthe heat-affected zone of steel plate 1) in said region by gouging ormachining.

A brittle crack developed in the butt-welded joint 2 propagates throughthe butt weld 5 or the heat-affected zone of the steel plate 1 whereresidual tensile stress is likely to occur. However, it was discoveredthat the propagation of the brittle crack is arrested by forming arepair weld 6 with a greater toughness than that of the butt weld 5 andan outer edge whose angle φ with respect to the longitudinal directionof the butt weld 5 is not less than 10 degrees and not more than 60degrees in a region to arrest the brittle crack, thereby diverting thebrittle crack from the butt weld 5 or the heat-affected zone of thesteel plate 1 to the base metal of the steel plate 1.

When this repair welding is applied to a welded structure consisting ofa vertical member (steel plate 1) and a horizontal steel stiffener(reinforcement) 3 intersectingly attached to a butt weld 5 thereon by afillet weld 4 as shown in FIG. 1, the effect described above is morepronounced because the repair weld 6 prevents the propagation of thebrittle crack by diverting it from the butt weld 5 or the heat-affectedzone of the steel plate 1 to the steel stiffener (reinforcement) 3.

Even in welded structures having no stiffener (reinforcement) 3 shown inFIG. 1, the repair weld 6 prevents the propagation of the brittle crackby diverting it from the butt weld 5 or the heat-affected zone of thesteel plate 1 to the steel plate 1. Therefore, the present invention isnot limited to the welded structure of the type shown in FIG. 1.

In the present invention, forming said repair weld 6 having excellenttoughness after removing ½ or more of plate thickness from either orboth sides of the butt-welded joint by gouging or machining favorablyarrests the propagation of crack more surely and increases resistance tobrittle crack propagation.

In the present invention, the brittle crack that has propagated throughthe butt weld 5 in the butt-welded joint 2 or the heat-affected zone ofthe steel plate 1 will pass into the repair weld 6 without diverting tothe steel plate 1 or stiffener 3 if, for example, the angle φ of theouter edge with respect to the longitudinal direction of the butt weldis relatively large. If, then, the roughness of the repair weld 6 islower than that of the butt weld 5, the crack having passed into therepair weld 6 does not stop but, instead, sometimes continues topropagate along the butt weld 5 in the butt-welded joint 2 or theheat-affected zone of the steel plate.

In the present invention, therefor, forming a repair weld 6 with suchexcellent toughness as at least 20° C. lower than the brittle-to-ductilefracture transition temperature vTrs of butt weld 5 by applying repairwelding with a welding material providing excellent fracture toughnessis preferable because such repair weld arrests the propagation of cracktherein even when the brittle crack having propagated along the buttweld 5 of the butt-welded joint 2 or the heat-affected zone of the steelplate 1 passes into the repair weld 6 without diverting to the steelplate 1 or stiffener 3.

In addition, the method to enhance the fracture toughness of the weldmetal in the repair weld 6 is not limited to any particular one. Thetoughness of the weld metal is controlled to said range of toughness byapplying repair welding with welding materials providing excellenttoughness such as a welding wire containing not less than Ni of 2 mass%.

FIG. 3 is a detail view of a repair weld made by the welding method ofthe present invention.

In FIG. 3, reference numerals 2, 5, 6 and 7 respectively designate abutt-welded joint, butt weld, repair weld and weld bead.

The inventors also made various empirical studies about optimumconditions for the repair weld. The studies revealed that forming saidrepair weld 6 with excellent toughness and making the angle φ of theouter edge thereof with respect to the longitudinal direction of thebutt weld not less than 10 degrees and not more than 60 degrees in aregion to arrest brittle crack in a butt-welded joint where brittlecrack is likely to propagate prevents the propagation of brittle crackin the base steel plate by diverting the brittle crack that wouldpropagate along the butt weld 5 of the heat-affected zone of the steelplate 1 to the steel plate 1 or stiffener 3. Even a brittle crack havingentered the repair weld 6 can be surely arrested therein.

The main technical idea of the present invention is to divert thebrittle crack that propagates along the longitudinal direction of thebutt weld 5, under the influence of the residual stress that occurs inthe vicinity of the outer edge of the repair weld 6, that is, in aregion where the butt-welded joint 2 and the repair weld 6 meet, fromthe butt weld 5 in the butt-welded joint 2 or the heat-affected zone ofthe steel plate 1 to the steel plate 1 or stiffener 3.

The inventors studied the conditions that make it possible to divert thebrittle crack propagating through the butt weld 5 in the butt-weldedjoint 2 or the heat-affected zone of the steel plate to the steel plate1 or stiffener 3 by varying the angle φ of the outer edge of the repairweld 6 with respect to the longitudinal direction of the butt weld 5.

The study led to a finding that if said angle φ exceeds 60 degreesbrittle fracture cannot be arrested unless the fracture toughness of therepair weld 6 is sufficiently high because brittle fracture often passesinto the repair weld 6. If said angle φ is less than 60 degrees, brittlefracture propagates along the boundary between the repair weld 6 andbutt-welded joint (base metal) and is arrested after being led to thesteel plate 1 or stiffener 3.

To get a fuller effect, it is preferable to make the angle φ is theouter edge of the butt weld with respect to the longitudinal directionof the butt weld not greater than 45 degrees.

However, if the angle φ is less than 10 degrees, the brittle crackpropagating along the boundary between the repair weld 6 and butt-weldedjoint 2 (base metal) approaches so close to the butt weld 5 afterpassing through the region of the repair weld 6 that the brittle cracksometimes re-propagates along the butt weld 5 or the heat-affected zoneof the steel plate 1. Therefore, the lower limit of the angle φ is setat 10 degrees.

It was also found that controlling the angle θ of the longitudinaldirection of at least the last layer of the repair weld bead 7 in therepair weld 6 with respect to the longitudinal direction of the buttweld 5 creates a large residual stress in a region where the repair weld6 and butt-welded joint (that is, the heat-affected zone of the steelplate 1) meet. By thus changing the direction of the principal stress inthe same region from the direction of the principal stress working onthe butt-welded joint 2 (that is, the butt weld 5 and the heat-affectedzone of the steel plate 1), the brittle crack propagating along saidbutt weld 5 in the butt-welded joint 2 or the heat-affected zone of thesteel plate 1 can be more stably diverted from the butt weld 5 or theheat-affected zone of the steel plate 1 and guided to the base metal.

Through various experiments the inventors discovered that the residualstress occurring in the region where the repair weld 6 and butt-weldedjoint 2 meet is greatly affected by the angle θ of the longitudinaldirection of at least the last layer of the repair weld bead 7 in therepair weld 6 with respect to the longitudinal direction of the buttweld 5.

When solidifying, the repair weld bead 7 tends to shrink more in thelongitudinal direction thereof, while the matrix of the butt-weldedjoint (that is, the heat-affected zone of the steel plate 1) near theend of the repair weld bead 7 does not deform much. As a consequence,residual stress occurs near the end of the repair weld bead 7.

Although a repair weld 6 having a large thickness is made by multi-passwelding, the residual stress having occurred near the end of the lastlayer of the repair weld bead 7 remains unaffected because no more passwill heat the last layer of the repair weld bead 7.

In order to create a great residual stress in the region where therepair weld 6 and butt-welded joint 2 meet, it was discovered that theangle θ of the longitudinal direction of at least the last layer of therepair weld bead 7 in the repair weld 6 with respect to the longitudinaldirection of the butt weld 5 plays an important role.

If said angle θ exceeds 80 degrees, the longitudinal direction of therepair weld bead 7 approaches the direction that is perpendicular to thelongitudinal direction of the butt weld 5 and, thus, agrees with orapproaches the direction of the principal component of the tensileresidual stress that occurs when the bead solidifies and shrinks andcauses the crack to propagate. Therefore, the brittle crack that haspropagated along the fusion line (FL) cannot be diverted to the vicinityof the repair weld.

It is therefore preferable to make the angle θ of at least the lastlayer of the repair weld bead 7 in the repair weld 6 with respect to thelongitudinal direction of the butt weld 6 not greater than 80 degrees.

It is preferable that the angle θ approaches 0 degree and the directionof the tensile residual stress occurring at the end of the repair weldbead 7 becomes perpendicular to the direction of the principal stressthat causes the crack to propagate because the resultant of thedirections works to arrest the rectilinear propagation of the crack andthereby enhances the effect of to divert the crack to the vicinity ofthe repair weld.

In order to create as great a residual stress as possible by forming arepair weld bead 7 in the repair weld 6, it is desirable to keep thetemperature difference between the repair weld bead 7 and the vicinityof the repair weld bead 7.

If the repair welding is performed with a large heat input, thetemperature in the vicinity of the repair weld bead 7 rises greatly,with the result that the repair weld bead 7 takes longer time to cool toapproximately room temperature and the residual stress decreases. Inorder to raise the residual stress, a smaller heat input is moreadvantageous.

EXAMPLE

Various tests were conducted to determine if repair welds made afterremoving part of butt-welded joints by gouging have abilities to arrestthe propagation of brittle cracks.

The tests were done to evaluate the abilities to arrest the rectilinearpropagation of brittle cracks along the longitudinal direction of thebutt weld. Test specimens were prepared by using steel plates, 2,500 mmsquare and having various thicknesses, and machining a dent having adepth approximately equal to half the thickness and a diameterapproximately equal to the thickness on each side of the specimen. Then,weld metals of various compositions and structures were made in saiddents by using welding materials of various compositions under variouswelding conditions as listed in Tables 1 to 4.

Then a V-notch cut to create a brittle crack by forcing in a wedge 8 wasmade so as to agree with the fusion line of the butt weld (that is, alarge-heat-input welded joint by electro gas welding) at a point 200 mmaway from the edge of each test specimen. The temperature at the centerof the test specimen was controlled to −10° C. by cooling the edgethereof to approximately −40° C. After applying a given stress, thewedge was driven in to said V-notch cut to create a brittle crack thatwas then allowed to propagate along the fusion line of the butt weld.

After the propagated brittle crack reached the repair weld, whether thesame brittle crack propagates further or not was evaluated.

Table 1 shows the results.

The toughness of the weld metal in the butt and repair welds weredetermined by taking specimens so that the longitudinal directionthereof is perpendicular to the longitudinal direction of the butt weld,applying a V-notch Charpy impact test to each specimen, and finding thebrittle-to-ductile fracture transition temperature vTrs (° C.). Thedifferences in vTrs (° C.) between the repair and butt welds shown inTable 1 are the differences thus determined.

The residual stress in the vicinity of the repair weld was determined byx-ray radioscopy at a point 2 mm closer to the welded base metal from apoint where the outer edges of the repair and butt welds meet (that is,the point 11 where the Charpy test specimen was taken, as indicated by ●in FIG. 3).

The direction of the determined residual stress is perpendicular to thelongitudinal direction of the butt weld and equal to the direction ofthe principal stress along which brittle crack propagates along the buttweld or the heat-affected zone of the steel plate.

The residual stresses (MPa) in the repair weld shown in Table 1 are theresidual stresses thus determined, with the negative sign − designatescompressive stress and the positive sign + designates tensile stress.

The point of brittle crack propagation shown in Table 1 means the pointat which the crack developed at the edge of the test specimen propagatesto the repair weld. FL (fusion line) indicates that the crack propagatedalong the fusion line of the repair weld and WM (weld metal) indicatesthat the crack propagated through the weld metal of the repair weld.

In the description of the resistance to brittle crack propagation inTable 1, “Diverted to and arrested in the base metal” means that thecrack diverted to outside the repair weld region and became arrested inthe base metal, without causing fracture and “Propagated after enteringthe weld metal (WM) and through the butt weld again” means that thecrack that has entered and propagated through the weld metal continuesto propagate through the butt weld, thereby causing fracture.

“Propagated again through the butt weld after propagating along therepair weld” means that the crack guided to the vicinity of the repairweld propagated through the repair weld and then along the butt weldagain.

“(Part of the crack branched and became arrested in the repair weld)”means that the crack branched and entered the repair weld because theresidual stress in the region where the repair and butt welds intersectwas not sufficiently compressive, though the principal component of thecrack was diverted and arrested in the base metal. The repair weld waspartly damaged but did not lead to fracture because the main componentof the crack was diverted to the base metal.

Test specimens Nos. 1 to 13 were examples of the present invention towhich repair welding was applied after removing part of the butt-weldedjoint according to the method of the present invention. All specimensshowed good resistance to crack propagation.

In test specimens Nos. 11, 12 and 13, residual stress did not reach thepredetermined level because said angle θ was too great. Although part ofthe crack, therefore, branched to the repair weld, the principalcomponent of the crack was diverted to and arrested in the base metal.

Test specimens Nos. 14 to 21 are for the purpose of comparison. In testspecimens Nos. 14 to 17, repair welding was done after removing part ofthe butt-welded joint. However, cracks entered and passed through therepair weld without being arrested therein, propagated along the buttweld again, and caused fracture because toughness differed littlebetween the repair and butt welds and the toughness in the repair weldwas low.

In test specimen No. 18, the value of the angle φ was too small thoughthe repair weld had adequate toughness. Therefore, crack propagatedalong the repair weld and, then, propagated through the butt weld againand caused fracture.

In test specimens Nos. 19 to 21, part of the butt-welded joint was notremoved before applying repair welding. Therefore, the brittle crackcreated in the butt-welded joint propagated therealong and broke thespecimen into exactly two halves. TABLE 1 Butt-welded Joint Plate RepairWeld Type Thickness Welding Welding Removing Width, Length, Total No. ofSteel (mm) Method Material Method W (mm) L (mm) Depth (mm) Examples 1YP40 50 EG EG-1 Gouging 115 138 40 of the 2 YP47 45 EG EG-60 Machining80 96 25 Invention 3 YP32 35 EG EG-1 Gouging 70 84 35 4 YP36 25 EG EG-3Gouging 60 72 20 5 YP40 50 EG EG-3 Machining 115 138 25 6 YP40 60 EGEG-3 Gouging 140 168 40 7 YP40 35 CO2 YM26 Gouging 80 96 20 8 YP47 70VEGA-II EG-60M Machining 150 180 50 9 YP32 40 SAW Y-Dx Gouging 90 108 25NB250H 10 YP40 80 VEGA-II EG-60M Gouging 150 180 50 11 YP36 20 FAB Y-DxGouging 50 60 10 NB250H 12 YP40 45 SAW Y-DMx Machining 100 120 35 NSH55E13 YP47 100 VEGA-II EG-60M Gouging 200 240 80 Examples 14 YP40 50 EGEG-3 Gouging 90 100 30 for 15 YP47 30 FCB Y-DMx Gouging 70 100 20Comparison NSH55E 16 YP32 45 EG EG-1 Machining 90 100 25 17 YP47 31 FCBY-DMx Gouging 70 100 20 NSH55E 18 YP40 25 FCB Y-DMx Machining 100 90 25NSH55E 19 YP36 65 VEGA EG-60M None — — 0 20 YP40 70 SEG DWS-1LG None — —0 21 YP40 60 VEGA-II EG-60M None — — 0 Repair Weld vTrs Difference Layerto Heat Welding Welding between Repair Angle φ Angle θ Control Input,No. Method Material and Butt Welds (Degree) (Degree) Angle θ HI (kJ/mm)Examples 1 CO₂ welding YM55H −20 12 0 All layers 2.5 of the 2 SMAW N-12−32 43 60 Last layer 2 Invention 3 CO₂ welding YM-3N −40 15 45 Lastlayer 2.5 4 CO₂ welding YM-1N −30 20 35 Last layer 3 5 SMAW YAWATA −15030 55 All layers 2.5 WELD B 6 CO₂ welding YM36E −25 45 45 All layers 5 7CO₂ welding YM55H −28 10 45 Last and 2 preceding 3 layers 8 SMAW YAWATA−180 35 40 Last and 2.9 WELD preceding B(M) 4 layers 9 CO₂ weldingYM309L −160 42 30 Last layer 5 10 CO₂ welding YM309L −100 55 45 Alllayers — 11 CO₂ welding YM316L −140 40 85 Last layer 2.8 12 SMAW YAWATA−175 45 82 Last layer 2.3 WELD B(M) 13 SMAW YM-3N −45 60 90 All layers —Examples 14 CO₂ welding YM28 −10 0 30 Last layer 3.5 for 15 SMAW L-60 4045 90 All layers 3.5 Comparison 16 CO₂ welding YM28 −5 80 45 All layers1.5 17 SMAW L-60 5 40 30 All layers 2.1 18 CO₂ welding YM28 −25 5 80Last layer 2.3 19 — None — 20 — None — 21 — None — Resistance to CrackPropagation Residual Stress in Repair Weld Propagation Point No. (MPa)in Repair Weld Results Examples 1 −350 FL Diverted to and arrested inthe base metal of the 2 −402 FL Diverted to and arrested in the basemetal Invention 3 −265 FL Diverted to and arrested in the base metal 4−300 FL Diverted to and arrested in the base metal 5 −340 FL Diverted toand arrested in the base metal 6 −300 FL Diverted to and arrested in thebase metal 7 −320 FL Diverted to and arrested in the base metal 8 −420FL Diverted to and arrested in the base metal 9 −300 FL Diverted to andarrested in the base metal 10 −200 FL Diverted to and arrested in thebase metal 11 100 FL(WM) Diverted to and arrested in the base metal(Part of the crack branched and became arrested in the repair weld) 12120 FL(WM) Diverted to and arrested in the base metal (Part of the crackbranched and became arrested in the repair weld) 13 300 FL(WM) Divertedto and arrested in the base metal (Part of the crack branched and becamearrested in the repair weld) Examples 14 100 WM Propagated again throughthe butt weld after propagating along the repair weld for 15 −200 WMPropagated after entering the weld metal (WM) and through the butt weldagain Comparison 16 300 WM Propagated after entering the weld metal (WM)and through the butt weld again 17 −250 WM Propagated after entering theweld metal (WM) and through the butt weld again 18 120 WM Propagatedagain through the butt weld after propagating along the repair weld 19 —— Propagated through the butt weld 20 — — Propagated through the buttweld 21 — — Propagated through the butt weldEG: Electro gas arc welding, CO₂: Carbon dioxide gas shield arc welding,SAW: Submerged arc welding, FCB: Copper backing one-side submerged arcwelding, FAB: Asbestos backing one-side submerged arc welding, VEGA:One-electrode oscillating electro gas welding, VEGA-II: Two-electrodeoscillating electro gas welding, SEG: Simplified electro gas welding,SMAW: Manual welding (Covered arc welding)

TABLE 2 Chemical compositions of steels (mass %) Type of Steel C Si Mn PS Ni Ti YP32 0.13 0.19 1.28 0.01 0.003 — 0.01 YP36 0.12 0.21 1.27 0.0070.004 — 0.01 YP40 0.11 0.21 1.3 0.006 0.003 — 0.01 YP47 0.08 0.24 1.220.007 0.002 1.02 0.01

TABLE 3 Chemical compositions of welding materials in butt-welds (mass%) Welding Material C Si Mn P S Mo Ni EG-1 0.1 0.33 1.45 0.015 0.01 0.27— EG-3 0.08 0.29 1.85 0.011 0.008 0.2 — EG-60 0.1 0.34 1.68 0.016 0.0150.29 — EG-60M 0.07 0.29 1.81 0.011 0.01 0.4 1.5 YM26 0.1 0.52 1.11 0.0170.011 — — Y-Dx 0.07 0.24 1.38 0.014 0.009 — — NB250H Y-DMx 0.1 0.25 1.400.014 0.009 0.36 — NSH55E DWS-1LG 0.06 0.20 1.51 0.015 0.01 — 2.0

TABLE 4 Chemical compositions of welding materials in repair welds (mass%) Welding Material C Si Mn P S Mo Ni Cr YM36E 0.05 0.33 1.2 0.004 0.002— — — YM55H 0.08 0.44 1.36 0.006 0.002 0.18 — — YM60C 0.07 0.38 1.380.005 0.012 0.35 — — YM-1N 0.05 0.39 1.25 0.007 0.005 0.22 0.98 — YM-3N0.04 0.3 0.7 0.006 0.004 — 3.56 — YAWATA 0.06 0.3 2.91 0.008 0.004 0.7668.5 16.7 WELD B YAWATA 0.09 0.24 3.27 0.008 0.003 2.32 65.1 15 WELDB(M) YM309L 0.022 0.47 1.52 0.02 0.007 — 13.1 24.1 YM316L 0.015 0.491.32 0.02 0.007 2.59 12.8 18.9 N-12 0.06 0.44 1.02 0.002 0.005 — 2.38 —N-13 0.05 0.42 0.46 0.01 0.008 — 3.35 — N-16 0.04 0.17 0.28 0.01 0.05 —6.65 — L-60 0.07 0.48 1.12 0.01 0.06 0.22 0.76 —

INDUSTRIAL APPLICABILITY

As described earlier, the present invention prevents the propagation ofbrittle cracks in the weld, even when such cracks have occurred inwelded joints, by applying specific repair welding to part ofbutt-welded joints.

The present invention, which thus provides welding methods for weldedstructures capable of preventing fatal fractures thereof, has remarkableindustrial advantages.

1. A welding method for manufacturing welded structures having excellentproperties to prevent the propagation of brittle fracture occurring inwelded joints, characterized by the step of forming a repair weld havinga greater toughness than that of a butt weld and an outer edge whoseangle φ, with respect to the longitudinal direction of the butt weld, isnot less than 10 degrees and not more than 60 degrees, by applyingrepair welding to a region to arrest a brittle crack in a butt-weldedjoint where a brittle crack is likely to propagate after removing partof the butt-welded joint in said region by gouging or machining.
 2. Thewelding method for manufacturing welded structures having excellentproperties to prevent the propagation of brittle fracture described inclaim 1, in which the toughness in said repair weld is at least 20° C.lower than the brittle-to-ductile fracture transition temperature vTrsof the butt-welded joint.
 3. The welding method for manufacturing weldedstructures having excellent properties to prevent the propagation ofbrittle fracture described in claim 1, in which said repair weld isprovided by applying repair welding to the pertinent region afterremoving ½ or more of plate thickness from either or both sides of thebutt-welded joint by gouging or machining.
 4. The welding method formanufacturing welded structures having excellent properties to preventthe propagation of brittle fracture described in claim 1, in whichcompressive residual stress not less than ½ of the yield stress of thewelded member is developed in a direction perpendicular to thelongitudinal direction of the butt weld in a region where said repairweld and butt-welded joint are in contact.
 5. The welding method formanufacturing welded structures having excellent properties to preventthe propagation of brittle fracture described in claim 1, in whichrepair welding is applied by controlling so that the angle θ of thelongitudinal direction of at least the last layer of the repair weldbead with respect to the longitudinal direction of the butt weld is notgreater than 80 degrees.
 6. The welding method for manufacturing weldedstructures having excellent properties to prevent the propagation ofbrittle fracture described in claim 1, in which repair welding isapplied by controlling so that the angle φ of the outer edge of the buttweld with respect to the longitudinal direction of the butt weld is notless than 10 degrees and not more than 45 degrees.
 7. A welded structurehaving excellent properties to prevent the propagation of brittlefracture occurring in welded joints, characterized by that the weldedstructure has a repair weld having a greater toughness than that of abutt weld and an outer edge whose angle φ with respect to thelongitudinal direction of the butt weld is not less than 10 degrees andnot more than 60 degrees in a region to arrest a brittle crack inbutt-welded joints where a brittle crack is likely to propagate.
 8. Thewelded structure having excellent properties to prevent the propagationof brittle fracture as described in claim 7, in which the toughness insaid repair weld is at least 20° C. lower than the brittle-to-ductilefracture transition temperature vTrs of the butt-welded joint.
 9. Thewelded structure having excellent properties to prevent the propagationof brittle fracture as described in claim 7, in which said repair weldis provided by applying repair welding to the pertinent region afterremoving ½ or more of plate thickness from either or both sides of thebutt-welded joint by gouging or machining.
 10. The welded structurehaving excellent properties to prevent the propagation of brittlefracture as described in claim 7, in which compressive residual stressnot less than ½ of the yield stress of the welded member is developed ina direction perpendicular to the longitudinal direction of the butt weldin a region where said repair weld and butt-welded joint are in contact.11. The welded structure having excellent properties to prevent thepropagation of brittle fracture as described in claim 7, in which theangle θ of the longitudinal direction of at least the last layer of therepair weld bead with respect to the longitudinal direction of the buttweld is not greater than 80 degrees.
 12. The welded structure havingexcellent properties to prevent the propagation of brittle fracture asdescribed in claim 7, in which the angle φ of the outer edge of the buttweld with respect to the longitudinal direction of the butt weld is notless than 10 degrees and not more than 45 degrees.